I saw a warning on the Internet that underarm
antiperspirants and deodorants may cause breast cancer. The warning
said that these products contain harmful substances, which can be
absorbed through the skin or enter the body through nicks caused
by shaving. Is this true?

No, this is a myth. The National Cancer Institute (NCI), the
nation's chief cancer research agency, is not aware of any evidence
of a link between use of antiperspirants and deodorants and the
development of breast cancer. Also, the U.S. Food and Drug Administration,
which regulates food, cosmetics, medicines, and medical devices,
has no evidence that ingredients in underarm antiperspirants or
deodorants cause cancer.

Source: "Ask the CIS" by the Cancer Information
Service. "Ask the CIS" is distributed by the Cancer Information
Service (CIS) of the Heartland, which serves Kansas, Nebraska, Missouri
& Illinois. The CIS is a program of the National Cancer Institute.
Call the CIS toll-free at 1-800-4-CANCER (1-800-422-6237) between
9 a.m. and 4:30 p.m. local time.